When a hydrocarbon well is drilled, a metal casing is placed in the well to line the wellbore. Additionally, space between the liner and the formation is often filled with cement. In order to connect the inside of the casing and wellbore with the inside of the formation, to allow for hydrocarbon flow from the formation to the inside of the casing, holes are formed through the casing and into the wellbore. This practice is commonly referred to as perforating of the casing and formation. Open-hole wells are also possible, i.e., where a casing is not used and perforation is directly applied to the formation.
Perforating guns are used to perforate a casing and wellbore. A perforating gun is often a long tubular device housing a number of shaped charges that face generally in a radial direction outward toward the casing and the formation. A loading tube commonly supports the shaped charges and is loading into an outer tubular shaped housing. The loading tube can take many shaped, i.e, a tube with openings for placement of the shaped charges, a flat plate that supports the shaped charges, etc.
The shaped charges generally have a cup shaped body and a conical shaped liner located in the opening of the cup. Explosive material is located between the inside of the cup shaped body and the liner so that upon detonation the liner is projected outward from the shaped charge, thereby penetrating the casing, cement and formation.
There are issues connected with this general operation of perforating, e.g., debris becoming located within the perforations in the formation as well as damage to the formation that affects permeability. These ideas are explained in more detail herein. Thus, it is desired to create a situation where debris in the formation perforations is limited and permeability of the perforated formation is improved. One way to address those issues is with underbalanced perforation, i.e., creating a low pressure inside the perforating gun thereby drawing in well fluids during the operation of perforating to suck debris into the gun/wellbore and away from the formation, thereby limiting the amount of debris in the formation perforations.
The embodiments in this application address a number of those issues connected therewith and improve on various aspects of underbalanced perforating.